Etched metal valve surfaces of tantalum or niobium or titanium



July 5, 1966 J, BURNHAM 3,259,475

ETCHED METAL VALVE SURFACES OF TANTALUM 0R NIOBIUM OR TITANIUM Jan/vamv/m M @fl/Ir, a

ETGHED METAL VALVE sURFAcEs oF TANTALUM 0R NIoBIUM on TITANIUM l FiledJan. 9. 1961 J. BURNHAM July 5, 1966 2 Sheets-Sheet 2 igina UnitedStatesPatent O 3,259,475 ETCHED METAL VALVE SURFACES F TANTA- LUM 0R NIOBIUM0R TITANIUM .lohn Burnham, 10960 Verano Road, West Los Angeles, Calif.

Original application Jan. 9, 1961, Ser. No. 81,396, now Patent No.3,190,822, dated `lune 22, 1965. Divided and this application May 3,1965, Ser. No. 452,621

This is a division of application Serial No. 81,396, filed January 9,1961, now Patent No. 3,190,822.

This invention pertains to etched surfaces of inert v-alve metalelectrodes.

In electrolytic capacitors valve metal electrodes are used because ofthe asymmetrical electrolytic characteristics of adherent oxide coatedsurfaces of such metals. It is established practice to etch suchelectrodes in order to increase their surface areas so as to make itpossible to use relatively small electrodes in order to obtainrelatively high capacitance ratings. 'Ihe term etch ratio is commonlyused in order to designate the proportionate amount by which theeffective surface areas of an electrode is increased by etching.

It is not considered necessary to lset forth in this specification thereasons why relatively inert valve metal electrodes, such as tantalum,niobium, and titanium electrodes are used in manufacturing electrolyticcapacitors instead of electrodes formed ofthe relatively more reactivemetal aluminum. Unfortunately inert valve metals as indicated abovecannot be etched satisfactorily using the established types of processesfor etching commonly employed with aluminum. As a consequence of this agreat deal of research and development activity `has been devoted to theetching of inert valve metals and in particular tantalum.

Such work has resulted in processes for electrolytic etching tantalum invarious types of electrolytes such as, for example, aqueous hydrouoricacid electrolytes and nonaqueous electrolytes in which solvents such asmethanol or formam-ide contain inorganic ionogens. Such processes areconsidered to require critical concentration control within theelectrolytes used in them. Frequently such concentrations are relativelydifficult to achieve and maintain. It is considered that prior etching.processes of these types for use with inert valve metals such astantalum v have not been successful in producing etched surfaces of auniform, satisfactory character having high etch ratios.

An object of this invention is to provide etched surfaces on inert valvemetals which are ldifferent from the etched surfaces provided on suchmetals by prior processes and which have comparatively high effectivesurface areas as indicated by etch ratio measurements.

A further object of this invention is to provide etched inert valvemetal electrodes which are physically strong, which have comparativelyhigh etch ratios and which may be easily created.

These Iand other objects of this invention as well as many specificadvantages of it will be apparent to those skilled in the eld of etchingcapacitor electrodes from a detailed consideration of the remainder ofthis specification including the appended claims and the accompanyingdrawings in which:

FIG. 1 is a curve indicating the effect of an electrolyte compositionchanges -in preparing an etched surface in accordance with thisinvention;

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FIG. 2 is a photomicrograph of an etched tantalum foil surface createdin accordance with this invention at 200 times its norm-al dimension;and

FIG. 3 -is a photomicrograph of an etched tantalum foil surface createdin .accordance with a prior etching process at 200 times its normaldimension.

As an aid to understanding this invention it may be summarily explainedas involving etched, inert valve metal surfaces as indicated in FIG. 2of the drawings, these surfaces having high effective areas as opposedto initial surface areas and having a grooved type of comparativelyuniform surface configuration. Such etched foils are, in accordance withthis invention, created by electrochemically etching these surfacesusing an electrolyte composition containing at le-ast one additive whichis considered to facilitate the removal of metal so as to achieve adesired type of etched surface configuration as indicated in FIG. 2 ofthe drawings,

Although this invention primarily pertains 'to etched tantalum foilelectrodes such as electrodes created from cold-rolled tantalum foil, itis not limited to Vthis type of electrode structure. This inventionwithin its broader scope encompasses inert valve metal electrodes of anyde-sired shape or configuration formed out of valve metals tantalum,niobium, and titanium and various alloys of these metals which are grainoriented and which are capable of being used in the same manner as thesemetals themselves in electrolytic capacitors as electrodes. Thus, thepresent invention encompasses not only metallic grain oriented foilelectrodes but other grain oriented electrode structures such as vetchedelectrodes having a cup-shaped appearance, etched wire electrodes oretched slug electrodes. With electrodes of various shapes as indicated,consider-able variations in electrode thicknesses will be encountered.As a general rule, it is preferred to utilize the present invention withcold-rolled metal foils such as tantalum foils having a thickness offrom about one-half mill to about one mill. Y

Any such electrodes having micro-crystalline grain oriented structuresare at least to a limited extent inuenced by manufacturing proceduresand methods. It is presently 'believed that many such electrodes, suchas inert valve metal foils, contain comparatively minute amounts ofsurface impurities which tend to affect the ability of such electrodesto be satisfactorily etched to high etch ratios. The presence of thesesurface impurities is normally detected when efforts are made to form oroxidize these foils at comparatively high voltages. While the precisenature of such impurities is not known, it is believed that they mayconsist of various surface oxides and compounds such as tantalumcarbide, gases such as hydrogen, oxygen or the like located within thecrystalline structures of electrode surfaces, and that theconcentrations of such impurities Will vary depending upon variations insuch crystalline structures. Further, the amounts of such impurities arepresently considered to be dependent upon not only manufacturingmethods, but upon conditions' to which such surfaces have been subjectedafter their manufacture.

In preparing the electrodes of this invention, in order to removeinsofar as is possible these surface impurities, an unetched foil ofsuch an inert valve metal may be iirst subjected to an initial cleaningstep if its surface is not already substantially free from surfaceimpurities. This cleaning step is considered to remove from its surfacesome of the valve metal itself; and more important than this, varioussurface impurities of the type indicated above. In carrying out thiscleaning step a valve metal, such as tantalum foil is made the anode inan electrolytic cell containing an inert cathode and a non-aqueouselectrolyte which includes either `a solvent or a solvent mixture and anon-film forming solute or solute mixture. The specific details of suchcleaning step are given in full in parent Patent No. 3,190,822,identified hereinabove, the entire disclosure of which is incorporatedherein by this reference.

A uniformly clean electrode surface is needed to facilitate thesubsequent etching so as to enable the production of the type of etchedsurface configuration indicated in FIG. 2 of the drawings.

The solutes used in the electrolyte employed for the cleaning'stepherein specified should be of non-film forming ionogen type of and, ofcourse, they should be soluble in the solvent or solvent mixtureemployed. Suitable solutes are compounds furnishing halide ions in thesolvents.

This cleaning step is in the general nature of a polishing step and, asindicated in the preceding, is primarily intended so as to removesurface metal containing impurities and not to accomplish any materialetching of an electrode surface. Because of this the conditions underwhich it may be carried out are primarily of a type well-known in theelectrolytic field. Thus, the temperature of this initial cleaning stepmay be varied in practice so as to encompass a range of temperaturesfrom about 20 C. to about Ythe boiling point of an electrolyte used. Ingeneral the higher the temperature the more rapidly metal is removedfrom an electrode surface. Satisfactory results can be v achieved atroom temperature.

Similarly the current and voltage used during this cleaning step may bevaried with comparatively wide limits. In general sufficient totalcharge should be used so as to remove substantially all surfaceimpurities. The amount of is preferred to etch with a combined A.C. andD.C. voltage where the R.M.S. value of the A.C. voltage component .isapproximately equal to the D C. voltage impressed.

However, it is possible to utilize during this cleaning or polishingstep square wave form currents and to periodically reverse the polarityin the electrolytic cell employed.

The voltages used during a cleaning or polishing operation as hereindescribed will vary depending upon the specific resistance of theelectrolyte employed as Well as the spacing of the anode and the cathodein any cell used. In general, the peak voltages should be held below avalue at which scintillation will occur. Similarly the current densitiesused may be varied within comparatively wide limits. In general Ithegreater the current density the greater the amount of metal removed andthe greater the amount of heating within an electrolytic cell.Satisfactory results have been achieved using current densities of fromabout 20 to about 60 milliamperes per square inch of electrode surfaceand voltages consisting of an A.C. half wave superimposed -upon a D C.voltage of equal magnitude to the magnitude of the R.M.S. value of theA.C. wave so that either the D.C. voltage or the R.M.S. value of theA.C. voltage used is from about 2.5 to about 125 volts.

Following the cleaning step indicated in the preceding discussion anelectrode in accordance with this invention is electrochemically etchedas the anode in a cell containing an vinert cathode and an electrolytecomposition separating these two electrodes. The preceding cleaning stepmay, of course, be omitted when foils capable of being dii invention.

satisfactory etch ratios and in obtaining the structure of thisinvention. Basically such an electrolyte is the same as the electrolyteas used during the cleaning or polishing step previously describedexcept for the addition of an additive which is capable of facilitatingthe removal of an inert valve metal from the surface of such a metalduring the electro-etching step. Because of this it is possible to carryout the invention by adding to an electrolyte as previously describeddesired quantities of one or more additive compounds and to carry on theetching in the same cell used for cleaning. It is also possible to carryout the process of this invention by physically transferring electrodesfrom one electrolytic cell to another so that the cleaning and etchingsteps are carried out 1n separate -cells instead of in the same cell.

Preferred additives used with the present invention are sulfoxidecomplexing agents which are soluble in anhydrous methanol or similarsolvents, which additions fall within the following general Iformulature or substituted derivatives of such compounds in which the ringsubstituted substituents are of an inert character or such as carbondisulfide are considered to be capable i of being used in achieving atleast some advantages in etching inert valve metals during this step ofthe present A mixture of complexing agents as well as a single suchagent can be employed in the etching electrolyte. Solubility of such anagent or agents in the solvent mixture employed is, of course, required.

The electrolyte used during this etching step may contain comparativelysmall or trace quantities of water. It is presently believed that up toabout 1,000 parts per million of water on the basis of the total solventor solvent mixture present does not detrimentally affect the characterof the etch obtained in accordance with this invention.

' Greater proportions of the water than this are preferably avoided inpracticing this invention inasmuch as such greater proportions of thiscontaminent tend to cause the preferred type of etched surface asindicated in FIG. 2

of the drawings. Similarly the presence of ketones such as acetoneshould be `avoided in an electrolyte used during this electro-etchingstep for substantially the same reasons. Also, various contaminants ofthis type tend to decrease to a substantial extent the etch ratios whichdetrimentally affect the etching process achieved in V comparativelywide limits.

' shown.

' weight of this additive.

accordance with this invention.

The amount of an additive as herein specified which may be added to -anelectrolyte may be varied within In `FIG. 1 of the drawings there isshown a curve indicating variation in etch ratios achievedwith'cold-rolled tantalum foils processes as described in the examplesindicated later in this specification. In this curve the affects of theaddition of dimethyl sulfoxide to a specific anhydrous methanolelectrolyte are The amounts of `dimethyl sulfoxide added to thiselectrolyte are shown on the basis of parts by weight. From this curveit will be seen that comparatively high etch ratios can be achieved bythe -addition of from about 0.5% by weight of this additive to about 70%by The effectiveness of the use of an additive of the type indicatedover this entire range is considered to be extremely significant.

This effectiveness is not completely understood at the present time. `Itis considered that an additive, such as dimethyl sulfoxide, used duringthe etching step of the present invention does not directly affect theprecise etching vreactions occurring directly upon an electrode surface,but that it indirect-ly -alects such reactions by tending to facilitatethe 4removal of inert valve metal from such a surface. It is believedthat in all probability an additive as herein described operates in theprocess of this invention so as to form a solvated complex with a valvemetal such as tantalum which is attacked during the etching operation.It is also believed that such a complex places such a valve metal into aform where it is removed from the innerfacial areas where electrodereactions take place, thus facilitating the etching operation. It isalso believed that water interferes with the etching reaction byhydrolyzing the complex ion `and forming an insoluble hydrated oxidewhich may coat the surface `and interfere with the ow of current tospecific sites -on the metal surface. It is presently believed thatcomparatively small trace-quantities of such contaminants effectivelyinhibit or prevent this type of action. This is shown in the point A inFIG. 1 indicating an electrolyte having a composition as shown by thecurve containing 4.2 parts by weight of water on the basis of the weightof the solvent employed. This water causes slightly in excess of a 50%drop of etchratio which could be achieved with this invention.

In general the temperature yand electrical conditions under which anelectro-etching step as herein specified was carried out are.substantially the same as the conditions used in carrying out anelectro-cleaning step as previously specified. For this reason it is notconsidered necessary to repeat the preceding discussion as totemperature, voltages, currents and the like. The amount of total chargeused during the etching will, of course, eiect the amount lof etchingachieved and, hence, will vary the etch ratio obtained in a nalelectrode. As a general rule it is desired to carry out theelectro-etching so as to achieve as high an etch ratio as possible andto use no more charge than is necessary in order to achieve such an etchratio since etching beyond a maximum etch ratio only serves to removemetal and, hence, serves to weaken the mechanical structure of anelectrode. The amount of total charge which would be used in etchingwill vary somewhat depending upon the nal results achieved with thisinvention. In general, however, it can be stated that satisfactory etchratios can be achieved by passing from about 160 to about 200 coulombsof charge per square inch of electrode surface etched.

It will, of course, be realized that the inal etch ratio achieved on anyelectrode surface is normally determined by 4me-ans of capacitancemeasurements taken after such a surface is `anodized in accordance withconventional practice. Depending upon the conditions of such anodizationthe etch ratio which is measured for any electrode will vary overcomparatively wide limits. Thus, for example, cold rolled tantalum foilswhich have been cleaned and etched in an identical manner in accordancewith this invention have been determined to have an etch ratio of about14 when formed in accordance with conventional practice at volts and ofabout 5 when formed at 180 volts under identical conditions except as tovoltage.

From kan examination of FIG. 2 of the drawings showing an etched surfacecreated in accordance with this invention it will be realized that theseetched sunfaces as herein described `are of a comparatively uniformlyetched character, and that when viewed under a microscope they have theessential appearance of a series of ridges and valleys, these ridges andvalleys being substantially regularly located parallel to one anotherand all having an unven surface configuration. This regularity and thedistance between successive ridges is determined by the neness of themetallographic grain and the direction of the valleys and ridges isdetermined by the direction of rolling of the metal and thus the grainorientation. This results from the fact that the rolling ope-rationbreaks down crystals into smaller ones land the fragments are laid outalong lines parallel to the rolling direction. Plastic deformation alsotakes place to elongate individual grain structures. The iineness of the'grain is of particular importance therefore to the ratios of surfacedeveloped on etching and metal having the maximum degree of cold workingby rolling, extruding or drawing is much to be preferred.

This type of etched surface is considered Vto be different in kind frometched surfaces of the general variety indicated in FIG. 3 of thedrawings created in accordance with known techniques. Etched surfacesmade ,by prior techniques on tantalum and other inert valve metals areof essentially la pook-marked configuration and they appear to beessentially smooth except where they are intersected by what may betermed holes extending into the metal surface. The smooth areas in priorart electrodes as indicated in FIG. 3 -are considered to be areas whereno significant increase in effective surface area has been achieved bythe etching process used in creating them. The smooth areas seen on thepriorI art electrode surface represent areas which have been essentiallyleft untouched by the electrochemical reaction and it Vis believed thatthese may be caused by very thin films of compounds of tantalum whichare resistant to etching and that with prior processes the etchingaction achieved primarily follow-s boundry lines between crystallineregions in the foil -being treated. The reduced etch ratios obtained inthe prior art thus appear to be due to the fact that not all the metalsurface comes into play. The amount of tantalum dissolved in forming theelectrode surfaces shown in FIGS. 2 and 3 was exactly the same. Anotheradvantage of the present invention is that it can be a very thin foil,as low as 0.2 mil, without perforation. Thus, strength of a foil inaccordance with this invention is greatly superior to a perforated priorfoil.

As an 4aid to understanding this invention the following specificexamples of carrying out .processes as herein described in creatingetched surfaces of the type indicated in FIG. 2 of the drawings aregiven. These examples are given by way of illustration only and are notconsidered as limiting this invention. For convenience of presentationthese examples are set forth in tabular form. All of the examplespertain to the treat-ment of commerci-ally available one-half inch widecold-rolled -tantalum foil. In 'all of these examples identicalelectrochemical cells are employed.

In all examples the etch ratios were determined after forming(oxidizing) :the etched electrodes indicated to 25 volts at anelectrolyte temperature of about 90 C. using 20 milliarnp. current. Thesame current was used in alll of the forming operations regardless ofelectrode size or the degree to which an electrode had been etched. Informing the electrodes in Example I the electrolyte employed contained:60 parts by weight ethylene glycol; 40 .parts by weight water; and 10parts by weight oxalic acid. In Example II the electrolyte contained 1part by weight ammonium phosphate .and 1000 parts by weight water.

ELECTROETCH Anhydrous methanol, solvent, parts by Weight 1. 000 1. 00

.Ammonium brom e, solu parts by Voltage, A.C.R.M.S. value mpressed onD.C. voltage 3.1 3.0

Current density, milliamps per sq. inch surface 40 40 Time, minutes 2020 Total charge, coulombs per s 100 100 Temperature, C. (initial) 20 20Etch ratio 9. 09 8. 8

I claim:

1. An etched surface of an inert valve metal selected from the groupconsisting of tantalum, niobium and titanium, said surface beingsubstantially free from surface impurities and having a surfaceappearance resembling a series of substantially parallel and regularlylocated ridges yand valleys, said ridges and valleys having an unevenetched `surface configuration.

2. A at, cold-rolled tantalum foil electrode, said electrode having anetched surface which is substantially free from surface impurities andwhich has a microscopic `appearance resembling a series of parallelsubstantially regularly located ridges and valleys, said ridges andvalleys having an uneven surface configuration and being substantiallyuniformly etched.

3. A wrought capacitor electrode of an inert valve Ymetal selected fromthe group consisting of tantalum, niobium and titanium, said wroughtelectrode being grain Oriented in accordance with the physical Workingresulting in the wrought form thereof, said wrought electrode having asurface, said surface being substantially free of surface impurities andbeing etched, said etched surface having the microscopic appearanceresembling a series of substantially parallel and substantiallyregularly spaced ridges and valleys, said ridges and valleys beingorganized with respect to the grain orientation of said wroughtelectrode, said ridges Iand valleys providing an uneven surfaceconfiguration and being substantially uniformly etched.

4. A wrought tantalum capacitor electrode, said wrought electrode beinggrain oriented in accordance with the physical working resulting in theWrought form thereof, said wrought electrode having a surface, saidsurface being substantially free of surface impurities and being etched,said etched surface having the microscopic appearance resembling alseries of substantially parallel and substantially regularly spacedridges and val-leys, said ridges and Valleys being organized withrespect to the grain orientation of said wrought electrode, said ridgesand valleys providing an uneven surface configuration and beingsubstantially uniformly etched.

5. An etched tantalum foil electrode having a surface configuration asshown in FIG. 2 of the drawing.

References Cited by the Examiner UNITED STATES PATENTS 2,742,416 4/1956Jenny 204--141 2,863,811 12/1958 Ruscetta et al 204-141 3,030,286 4/1962Tao 204-141 3,070,522 12/1962 Robinson et al 204-141 DAVID L. RECK,Primary Examiner.

R. O. DEAN, Assistant Exa'mner.

1. AN ETCHED SURFACE OF AN INERT VALVE METAL SELECTED FROM THE GROUPCONSISTING OF TANTALUM, NIOBIUM AND TITANIUM, SAID SURFACE BEINGSUBSTANTIALLY FREE FROM SURFACE IMPURITIES AND HAVING A SURFACEAPPEARANCE RESEMBLING A SERIES OF SUBSTANTIALLY PARALLEL AND REGULARLOCATED RIDGES AND VALLEYS, SAID RIDGE AND VALLEYS HAVING AN UNEVENETCHED SURFACE CONFIGURATION.